For Calibre to actually have the ability to combine books in a series with a clickable TOC is a great idea.

I currently use Calibre to convert each file in a series to rtf format, open in OpenOffice, save as HTML - combine all into a single HTML file and then do a large amount of cleaning up as well as coding in a clickable TOC. It's a lengthy process but once I have the HTML format I can easily convert to mobi or other formats. I guess I could use Sigil but I like HTML.